We investigate the effects of preheating temperature on the crystalline state of seed layer and hydrothermal ZnO nanorods grown thereon. The ZnO seed layers are deposited on Si (100) substrates by sol-gel multi-spin coatings and preheated at different elevated temperatures. Average grain size of ZnO seeds measured by scanning electron microscopy at each temperature of 100, 200, and 300 degrees C is about 29, 21, and 19 nm, respectively, and the more intense (002) reflection by X-ray diffraction (XRD) and higher ultra-violet emission peak from photoluminescence are observed at lower preheating temperature. The seed layers also exhibit a tendency of increase in oxygen vacancy concentration due to the increase in crystal defects present in the thin film with the increase of preheating temperature. The nanorod crystals grown by hydrothermal method exhibit a strong dependence of growth alignment along (002) orientation of the seed layer influenced by the preheating temperature as revealed by XRD analysis after nanorod growth. (C) 2015 Elsevier B.V. All rights reserved.